Dusan Stevanovic

Strategies for improving performance of IEEE 802.15.4/ZigBee WSNs with path-constrained mobile sink(s)

Most of the existing works on the topic of sink mobility in wireless sensor networks (WSNs) are of purely theoretical nature. The aim of this paper is to discuss the challenges as well as potential benefits associated with the use of mobile sinks in WSNs that operate in space-constrained environments and employ real-world technology. Specifically, we examine the pros and cons of deploying path-constrained sink mobility in the framework of IEEE 802.15.4/ZigBee enabled sensor networks. The main contributions of this paper are as follows: First, we demonstrate that the advantages of deploying path-constrained sink mobility, as identified in one of our earlier works [4], are not fully applicable to ZigBee WSNs. Specifically, our OPNET-based simulation study shows that in ZigBee WSNs the findings from [4] hold only conceptually, at the highest level of user-data routing. However, once all of the mobility-related overhead is accounted for, no actual benefit of deploying a mobile-over deploying a static-sink can be observed. Subsequently, we propose the use of three mechanisms for control of mobility-related overhead in ZigBee WSNs: Suppressed Route Discover, Node Association Based on Residual Energy, and Footprint Chaining. The most complex of the three mechanisms (Footprint Chaining) is studied in detail, and conditions under which this technique achieves optimal performance are precisely identified. The presented simulation results prove that with the three proposed mechanisms in place the benefits of mobile-over static-sink deployment can be regained, almost to the same extent as theoretically identified in [4]. To our knowledge, this paper is one of the first attempts to bring the topics of path-constrained sink mobility and ZigBee standard together. It is also the first published work to propose improvements to the current ZigBee standard specifically targeted for WSNs that involve the use of mobile sinks.

Unsupervised Clustering of Web Sessions to Detect Malicious and Non-malicious Website Users

Abstract Denial of Service (DoS) attacks are recognized as one of the most damaging attacks on the Internet security today. Recently, malicious web crawlers have been used to execute automated DoS attacks on web sites across the WWW. In this study, we examine the use of two unsupervised neural network (NN) learning algorithms for the purpose web-log analysis: the Self- Organizing Map (SOM) and Modified Adaptive Resonance Theory 2 (Modified ART2). In particular, through the use of SOM and Modified ART2, our work aims to obtain a better insight into the types and distribution of visitors to a public web-site based on their link-traversal behaviour, as well as to investigate the relative differences and/or similarities between malicious web crawlers and other non-malicious visitor groups. The results of our study show that, even though there is a pretty clear separation between malicious web-crawlers and other visitor groups, around 8% of malicious crawlers exhibit very ‘human-like’ browsing behaviour and as such pose a particular challenge for future web-site security systems.

Performance of IEEE 802.15.4 in wireless sensor networks with a mobile sink implementing various mobility strategies

In this work, we investigate the advantages and challenges of deploying a single mobile sink in IEEE 802.15.4/ZigBee wireless sensor networks (WSNs). The first part of the paper provides an overview of the most recent research on sink mobility in WSNs, placing a special emphasis on different types of sink mobility (random, predictable and controlled) and discussing the application scenarios most suitable for their respective deployment. In the second part of the paper, our OPNET model for simulation of large-scale and ZigBee-based wireless sensor networks is presented. The model enables effective evaluation of random and predictable sink mobility under varying conditions and forms of routing in the underlying ZigBee WSN. The results obtained using this model show that in terms of energy efficiency ZigBeepsilas tree-based routing outperforms ZigBeepsilas mesh routing, both in the case of random and predictable sink mobility. At the same time, under both mobility models, tree-based routing generates longer delays in the delivery of data reporting packets. Furthermore, when compared against each other assuming identical network conditions, random mobility is shown to achieve higher energy efficiency and shorter packet delays than predictable mobility.

Sink mobility in wireless sensor networks: When theory meets reality

The use of sink mobility in wireless sensor networks (WSN) is commonly recognized as one of the most effective means of load balancing, ultimately leading to fewer failed nodes and longer network lifetime.

Performance Analysis of ZigBee-Based Wireless Sensor Networks with Path-Constrained Mobile Sink(s)

In the wireless sensor network (WSN) literature, the use of a mobile sink is commonly viewed as one of the most successful means of load balancing as well as an effective defense against the so-called hot-spot phenomenon. The aim of this paper is to investigate the real-world applicability of the known theoretical benefits associated with the use of mobile sink(s). In particular, we examine the pros and cons of deploying path-constrained mobile sink(s) in IEEE 802.15.4 / ZigBee-based WSNs.The main contributions of this paper are as follows: First, analytically and through simulation, we demonstrate that in idealistic (zero-overhead) networks the use of a mobile sink does result in a more even distribution of routing load and longer network lifetime, as earlier suggested in the WSN literature. Moreover, in small- to medium- size zero-overhead WSNs, the outer-peripheral and the diagonal-cross trajectory appear to be more effective than other types of mobile-sink trajectories. Unfortunately, real-world networks, including ZigBee WSNs, are not zero-overhead – these networks employ special mechanisms that generate additional (overhead) traffic in order to manage congestion and node mobility. The results of our OPNET-base simulation study demonstrate that in IEEE 802.15.4 / ZigBee WSNs, once all of the overhead traffic is accounted for, the theoretical advantage of deploying a mobile- vs. deploying a static- sink completely disappears. Hence, for anybody contemplating the use of a mobile sink in ZigBee sensor networks, the minimization of protocol overhead may have to be the first course of action. In the last part of the paper, we introduce two simple mechanisms for reduction of mobility-related overhead in ZigBee WSNs. The presented simulation results suggest that with these mechanisms in place the superiority of mobile- over static- sink deployment can be regained.

Next Generation Application-Layer DDoS Defences: Applying the Concepts of Outlier Detection in Data Streams with Concept Drift

The existing state-of-the art in the field of application-layer DDoS protection is generally designed, and thus effective, only for static Web-domains. To the best of our knowledge, this paper is the first one to study the problem of application-layer DDoS defense in Web-sites of dynamic content and/or organization and under non-trivial bot (i.e., Attack) behavior. The main contributions of the paper are threefold: 1) we provide a detailed taxonomy of the existing and next-generation application-layer HTTP-based DDoS attacks, 2) we discuss the relevance of a branch of data mining theory -- known as data streams with concept drift -- to the problem of application-layer DDoS defense in dynamic Web-domains, 3) we present the outline of our next-generation anti-DDoS system that is intended for dynamic Web-domains facing different sophisticated variants of application-layer DDoS attacks. The paper also includes some of our preliminary experimental results concerning the detection of malicious Web-users/sessions using the proposed system.

Detecting web crawlers from web server access logs with data mining classifiers

In this study, we introduce two novel features: the consecutive sequential request ratio and standard deviation of page request depth, for improving the accuracy of malicious and non-malicious web crawler classification from static web server access logs with traditional data mining classifiers. In the first experiment we evaluate the new features on the classification of known well-behaved web crawlers and human visitors. In the second experiment we evaluate the new features on the classification of malicious web crawlers, unknown visitors, well-behaved crawlers and human visitors. The classification performance is evaluated in terms of classification accuracy, and F1 score. The experimental results demonstrate the potential of the two new features to improve the accuracy of data mining classifiers in identifying malicious and well-behaved web crawler sessions.

Sink mobility in wireless sensor networks: a (mis)match between theory and practice

In the wireless sensor network (WSN) literature, the use of a mobile sink is commonly viewed as one of the most successful means of load balancing and is often recommended as an effective defence against the so-called hot-spot phenomenon. In this paper, we investigate the real-world applicability of theoretical findings concerning sink mobility. The main contributions of the paper include: First, we analytically demonstrate that in small- to mid- size square-shaped WSNs implementing virtual grid topology the (outer) periphery is not necessarily the best performing mobile-sink trajectory, as earlier suggested in [10]. In such networks, the diagonal-cross appears to be at least as effective as the outer peripheral trajectory. Second, our OPNET-based study of IEEE 802.15.4 / ZigBee WSNs suggests that in these networks, once all of the protocol overhead is accounted for, no actual benefits of deploying a mobile- over deploying a static- sink can observed. Hence, for anybody contemplating the use of a mobile sink in ZigBee-based sensor networks, the minimization of protocol overhead may have to be the first course of action.

A Simulation-Based Performance Analysis of Various Multipath Routing Techniques in ZigBee Sensor Networks

Multipath routing is generally known for its highly effective performance in applications involving data streaming. However, to date, only a handful of research studies have looked at the performance of multipath routing in WSN systems. Moreover, according to our knowledge, there has been no earlier study on the use of multipath routing in ZigBee WSNs.

Application-layer DDoS in dynamic Web-domains: Building defenses against next-generation attack behavior

Current state-of-the art in the field of application-layer DDoS protection is generally designed, and thus effective, only for static Web-domains. To the best of our knowledge, our work is the first one to study the problem of application-layer DDoS defense in Web-domains of dynamic content and/or organization, and under next-generation bot/attack behavior. The main contributions of this paper are twofold: (1) we discuss the relevance of a branch of data mining theory - known as data streams with concept drift - to the problem of application-layer DDoS defense in dynamic Web-domains, (2) we present the outline of our next-generation anti-DDoS system that is intended for dynamic Web-domains facing different sophisticated variants of application-layer DDoS attacks.